There isn't an average, and most people of the people you ask will say a number, high or lower than the real one, to fit in with society's idea of how many partners you should not have. Personally, at the age of 25, i have no idea how many people I've been intimate with. I just never bothered to start counting, because it's not that important. For the most part, the only reason why a partner would even ask is to find out how many people they will potentially compared too. And that has nothing to do with how many you've been with, it's about your partner being unsure of themselves.